Photovoltaic voltage regulation

ABSTRACT

A photovoltaic system includes: a photovoltaic generator comprising strings that each includes one or more photovoltaic cells; a power converter; switches; and a controller. The power converter is configured to convert direct current (DC) power provided by the photovoltaic generator into alternating current (AC) power, and to output the AC power. Each switch is associated with one of the strings and is configured to connect the associated string to the power converter when set to a first setting, such that power generated by the first string can flow to the power converter. Each switch is also configured to disconnect the string from the power converter when set to a second setting. The controller is configured to control the power provided by the photovoltaic generator by selectively connecting the strings of the photovoltaic generator to the power converter by controlling the settings of the switches.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/870,306[now U.S. Pat. No. 9,941,701], filed Sep. 30, 2015, and entitled“PHOTOVOLTAIC VOLTAGE REGULATION,” which is a divisional of U.S.application Ser. No. 13/152,787 [now U.S. Pat. No. 9,184,594], filedJun. 3, 2011, and entitled “PHOTOVOLTAIC VOLTAGE REGULATION,” each ofwhich is hereby incorporated by reference.

BACKGROUND

Photovoltaic systems use solar cells to convert light into electricity.A typical photovoltaic system includes several components, includingphotovoltaic cells, mechanical and electrical connections, mountings,and controllers for regulating and/or modifying the electrical currentproduced by the photovoltaic system.

The following terms are used herein to describe various componentsand/or operational aspects of photovoltaic systems:

PV photovoltaic DC direct current AC alternate current V_(OC) opencircuit voltage V_(GRID) grid voltage V_(NOM) nominal grid voltageI_(GRID) grid current

FIG. 1 is a functional block diagram of a typical PV system 100. Thephotovoltaic system 100 includes a photovoltaic generator 101 thatconverts sunlight into electricity. In a conventional PV system, such asthe photovoltaic system 100, the voltage generated by the system can becontrolled by extracting an appropriate amount of power from the PVgenerator 101, passing the power from the PV generator 101 to a powerconverter 102 through to a power sink 103. According to an embodiment,the power converter 102 can comprise an electronic power converter. In atypical implementation, the power sink 103 is the electrical power grid(sometimes also referred to as the power “mains”). The grid comprises anelectrical network for generating, transmitting, controlling, anddistributing power from power generators to power consumers at variousservice locations across the network. The power converter 102 convertsDC power provided by the PV generator 101 into AC power that can bedistributed on the grid.

FIG. 2 is a more detailed block diagram of a conventional PV powersystem 200 that can be used to implement the system illustrated inFIG. 1. The PV power system 200 includes a solar cell array 201 thatcomprises solar cells (also referred to as photovoltaic cells). Thesolar cells are solid state devices that convert the energy of sunlightdirectly into electricity by the photovoltaic effect. The solar cellsgenerate DC voltage.

The solar cell array 201 is coupled to a DC switch 202. The DC switch202 can be closed to connect the solar cell array 201 to DC capacitorbank 204, or opened to disconnect the solar cell array 201 from the DCcapacitor bank 204. When the DC switch 202 is closed and the solar cellarray 201 is generating power, the solar cell array 201 can providepower to charge the DC capacitor bank 204. The DC capacitor bank 204 isalso connected to an inverter 205.

The inverter 205 converts the DC voltage output from the capacitor bank204 into a 3-phase (or in some cases 2-phase) pulsed AC voltage. Theinverter 205 outputs pulsed AC current to a filter 206. The filter 206converts the pulsed AC current output by the inverter 205 into asinusoidal AC voltage. The sinusoidal AC voltage can then be output to amains power grid 209. If an AC mains switch 207 is closed, thesinusoidal AC voltage output by the filter 206 is received by the powertransformer 208. The power transformer 208 adapts the voltage output bythe PV system 200 to the grid voltage. This configuration allows the PVsystem 200 to output electricity onto the mains grid 209. The voltageoutput by the photovoltaic system 200 is no higher than the gridvoltage.

Controlling the voltage generated by a PV generator such as the solarcell array 201 is important because it can help to (a) increase thepower generated by the solar panels, and (b) reduce the voltage stresson the power converter. If the power sink 103, such as the grid 209, isunable to absorb the available power produced by the PV generator 101,the PV voltage will increase toward the open circuit level (V_(OC)) andwill ultimately produce an increased voltage stress on the powerconverter 102. In conventional systems, this is addressed by“overdesigning” the power converter, such that the power converter 101can reliably operate with the PV open circuit voltage levels.Overdesigned systems have lower efficiency and higher complexity thansystems that are not overdesigned.

FIG. 3 illustrates an alternative approach that conventional systemshave used to address these issues. A PV power system 300 includes apre-load 304 in the form of a dissipative resistive load parallel to aPV generator 301. In the event that a power sink 303 is unable to absorbthe power generated by the PV generator 301, the pre-load 304 can beactivated to supplement the power sink and to maintain the PV voltage atlevels that are safe for power converter 302. The use of a pre-load 304,however, can be prohibitively expensive and can pose a fire risk.

SUMMARY

Techniques are described for regulating the voltage generated by aphotovoltaic system. For example, a photovoltaic system includes aphotovoltaic generator that includes photovoltaic cells arranged instrings. A configurable string controller can detect events where thevoltage produced by the photovoltaic generator should be regulated andselectively connect or disconnect the strings to regulate the voltageprovided by the photovoltaic generator.

An example of a photovoltaic system includes: a photovoltaic generatorincludes strings that each includes one or more photovoltaic cells; apower converter; switches; and a controller. The power converter isconfigured to convert direct current (DC) power provided by thephotovoltaic generator into alternating current (AC) power, and tooutput the AC power. Each switch is associated with one of the stringsand is configured to connect the associated string to the powerconverter when set to a first setting, such that power generated by thefirst string can flow to the power converter. Each switch is alsoconfigured to disconnect the string from the power converter when set toa second setting. The controller is configured to control the powerprovided by the photovoltaic generator by selectively connecting thestrings of the photovoltaic generator to the power converter bycontrolling the settings of the switches.

Implementation of the photovoltaic system may include one or more of thefollowing features. The controller is configured to monitor the voltageof a power sink, and the controller is configured to decrease the powerprovided by the photovoltaic generator to the power converter byselectively disconnecting strings of the photovoltaic generator inresponse to a decrease in voltage of the power sink. The controller isconfigured to increase the power provided by the photovoltaic generatorto the power converter by selectively disconnecting strings of thephotovoltaic generator in response to an increase voltage of the powersink. The controller includes a tangible, non-transitorycomputer-readable memory, modules comprising processor executable codestored in the memory, a processor connected to the memory and configuredto access the modules stored in the memory, and a control interfaceconfigured to send control signals to the switches. The modules includea voltage control module, a string selection module, and a controlsignal module. The voltage control module is configured to cause theprocessor to: monitor the voltage of the power sink to identify changesin the voltage of the power sink and to determine whether to connect ordisconnect one or more strings of the photovoltaic generator, to adjustthe power provided by the photovoltaic generator, in response to achange in voltage of the power sink. The string selection module isconfigured to cause the processor to select one or more strings of thephotovoltaic generator in response to be connected or disconnected basedon a determination by the voltage control module that one or morestrings of the photovoltaic generator should be connected ordisconnected in response to a change in voltage of the power sink. Thecontrol signal module is configured to cause the processor to sendcontrol signals to the switches to cause the one or more strings to beconnected to the power converter or to be disconnected from the powerconverter. The controller is further configured to: receive an inverterstartup signal indicating that the power converter is in a startupperiod during which power provided by the photovoltaic generator is tobe gradually ramped up, to disconnect any strings in excess of stringsused to provide startup voltage, and to iteratively connect strings togradually increase power provided by the photovoltaic generator.

An example of a method for controlling the power output of aphotovoltaic system includes: receiving an inverter startup signalindicating that a power converter of the photovoltaic system is in astartup period during which power provided by the photovoltaic generatoris to be gradually ramped up; disconnecting any strings in excess ofstrings used to provide startup voltage; and iteratively connectingstrings to gradually increase power provided by the photovoltaicgenerator. The photovoltaic system includes a photovoltaic generatorthat includes strings where each string includes one or morephotovoltaic cells.

Implementations of such a method may include one or more of thefollowing features. Each string is associated with a switch, anddisconnecting any strings in excess of strings used to provide startupvoltage include sending a control signal to each of the switches,associated with each of the strings to be disconnected, to disconnectthe strings from the power converter. Connecting the strings to adjustthe photovoltaic voltage provided by the photovoltaic generator includessending a control signal to each of the switches, associated with eachof the strings to be connected, to connect the strings to the powerconverter.

An example of a method for controlling the power output of aphotovoltaic system is includes: monitoring a voltage of a power sinkassociated with the photovoltaic system; determining whether the voltageof the power sink has decreased, and in response to the voltage of thepower sink decreasing: calculating a percentage of the voltage of thepower sink relative to a nominal level associated with the power sink,the nominal level representing a desired voltage level for the powersink, calculating a number of strings of the photovoltaic generator tobe disconnected to decrease a photovoltaic voltage provided by thephotovoltaic generator, wherein disconnecting the string prevents powergenerated by the string from reaching a power converter of thephotovoltaic system that converts direct current (DC) power toalternating current (AC) power expected by the power sink, anddisconnecting the calculated number of strings to adjust thephotovoltaic voltage provided by the photovoltaic generator. Thephotovoltaic system photovoltaic system includes a photovoltaicgenerator that includes strings where each string includes one or morephotovoltaic cells.

Implementations of such a method may include one or more of thefollowing features. Determining whether the voltage of the power sinkhas increased, and in response to the voltage of the power sinkincreasing: calculating a percentage of the voltage of the power sinkrelative to a nominal level associated with the power sink, the nominallevel representing a desired voltage level for the power sink, andcalculating a number of strings of the photovoltaic generator to beconnected to decrease a photovoltaic voltage provided by thephotovoltaic generator, where connecting the string allows powergenerated by the string to reach the power converter of the photovoltaicsystem, and connecting the calculated number of strings to increase thephotovoltaic voltage provided by the photovoltaic generator. The voltageof the power sink decreases as a result of a low voltage ride through(LVRT) event, and reducing the voltage of the photovoltaic generator byan amount proportional to the decrease in voltage of the power sink. Thevoltage of the power sink increases after the LRVT event, and increasingthe voltage provided by the photovoltaic generator by an amountproportional to the increase in voltage of the power sink. Each stringis associated with a switch, and where disconnecting the calculatednumber of strings to adjust the photovoltaic voltage provided by thephotovoltaic generator includes sending a control signal to the switchesassociated with each of the strings to be disconnected to disconnect thestrings from the power converter. Connecting the calculated number ofstrings to adjust the photovoltaic voltage provided by the photovoltaicgenerator includes sending a control signal to the switches associatedwith each of the strings to be connected to connect the strings to thepower converter.

An example of a method for controlling the power output of aphotovoltaic system includes: determining a voltage of a power sinkassociated with the photovoltaic system; determining a reference voltagefor the photovoltaic generator, the reference voltage representing adesired voltage for the photovoltaic generator; determining a currentvoltage for the photovoltaic generator; determining a difference betweenthe reference voltage and the current voltage; calculating a number ofstrings of the photovoltaic generator to connect or disconnect based onthe difference, wherein disconnecting the string prevents powergenerated by the string from reaching a power converter of thephotovoltaic system that converts direct current (DC) power toalternating current (AC) power expected by the power sink. Connectingthe string allows power generated by the string to reach the powerconverter of the photovoltaic system. The method also includesconnecting or disconnecting the calculated number of strings to adjustthe photovoltaic voltage provided by the photovoltaic generator. Thephotovoltaic system photovoltaic system includes a photovoltaicgenerator that includes strings where each string includes one or morephotovoltaic cells.

Implementations of such a method may include one or more of thefollowing features. Each string is associated with a switch, and wheredisconnecting the strings to adjust the photovoltaic voltage provided bythe photovoltaic generator includes sending a control signal to theswitches associated with each of the strings to be disconnected todisconnect the strings from the power converter. Connecting the stringsto adjust the photovoltaic voltage provided by the photovoltaicgenerator includes sending a control signal to the switches associatedwith each of the strings to be connected to connect the strings to thepower converter.

An example system for controlling the power output of a photovoltaicsystem includes a photovoltaic generator that includes strings whereeach string includes one or more photovoltaic cells. The system includesmeans for receiving an inverter startup signal indicating that a powerconverter of the photovoltaic system is in a startup period during whichpower provided by the photovoltaic generator is to be gradually rampedup; means for disconnecting any strings in excess of strings used toprovide startup voltage; and means for iteratively connecting strings togradually increase power provided by the photovoltaic generator.

Implementations of the system for controlling the power output of aphotovoltaic system may include one or more of the following features.Each string is associated with a switch, and the means for disconnectingany strings in excess of strings used to provide startup voltage furthercomprises means for sending a control signal to each of the switchesassociated with each of the strings to be disconnected to disconnect thestrings from the power converter. The means for connecting the stringsto adjust the photovoltaic voltage provided by the photovoltaicgenerator further comprises means for sending a control signal to eachof the switches associated with each of the strings to be connected toconnect the strings to the power converter.

An example system for controlling the power output of a photovoltaicsystem includes a photovoltaic generator that includes strings whereeach string includes one or more photovoltaic cells. The system includesmeans for monitoring a voltage of a power sink associated with thephotovoltaic system; means for determining whether the voltage of thepower sink has decreased, and in response to the voltage of the powersink decreasing: means for calculating a percentage of the voltage ofthe power sink relative to a nominal level associated with the powersink, the nominal level representing a desired voltage level for thepower sink, means for calculating a number strings of the photovoltaicgenerator to be disconnected to decrease a photovoltaic voltage providedby the photovoltaic generator, wherein disconnecting the string preventspower generated by the string from reaching a power converter of thephotovoltaic system that converts direct current (DC) power toalternating current (AC) power expected by the power sink, and means fordisconnecting the calculated number of strings to adjust thephotovoltaic voltage provided by the photovoltaic generator.

Implementations of the system for controlling the power output of aphotovoltaic system may include one or more of the following features.The system includes means for determining whether the voltage of thepower sink has increased, and in response to the voltage of the powersink increasing the means for determining whether the voltage of thepower sink has increased includes: means for calculating a percentage ofthe voltage of the power sink relative to a nominal level associatedwith the power sink, the nominal level representing a desired voltagelevel for the power sink; means for calculating a number strings of thephotovoltaic generator to be connected to decrease a photovoltaicvoltage provided by the photovoltaic generator, wherein connecting thestring allows power generated by the string to reach the power converterof the photovoltaic system; and means for connecting the calculatednumber of strings to increase the photovoltaic voltage provided by thephotovoltaic generator. The voltage of the power sink decreases as aresult of a low voltage ride through (LVRT) event, and the systemincludes means for reducing the voltage of the photovoltaic generator byan amount proportional to the decrease in voltage of the power sink. Thevoltage of the power sink increases after the LRVT event, and the systemincludes means for increasing the voltage provided by the photovoltaicgenerator by an amount proportional to the increase in voltage of thepower sink. Each string is associated with a switch, and the means fordisconnecting the calculated number of strings to adjust thephotovoltaic voltage provided by the photovoltaic generator includesmeans for sending a control signal to the switches associated with eachof the strings to be disconnected to disconnect the strings from thepower converter. The means for connecting the calculated number ofstrings to adjust the photovoltaic voltage provided by the photovoltaicgenerator further comprises means for sending a control signal to theswitches associated with each of the strings to be connected to connectthe strings to the power converter.

An example system for controlling the power output of a photovoltaicsystem includes a photovoltaic generator that includes strings whereeach string includes one or more photovoltaic cells. The system includesmeans for determining a voltage of a power sink associated with thephotovoltaic system; means for determining a reference voltage for thephotovoltaic generator, the reference voltage representing a desiredvoltage for the photovoltaic generator; means for determining a currentvoltage for the photovoltaic generator; means for determining adifference between the reference voltage and the current voltage; meansfor calculating a number of strings of the photovoltaic generator toconnect or disconnect based on the difference, wherein disconnecting thestring prevents power generated by the string from reaching a powerconverter of the photovoltaic system that converts direct current (DC)power to alternating current (AC) power expected by the power sink, andwherein connecting the string allows power generated by the string toreach the power converter of the photovoltaic system; and means forconnecting or disconnecting the calculated number of strings to adjustthe photovoltaic voltage provided by the photovoltaic generator.

Implementations of the system for controlling the power output of aphotovoltaic system may include one or more of the following features.Each string is associated with a switch, and the means for disconnectingthe strings to adjust the photovoltaic voltage provided by thephotovoltaic generator comprises means for sending a control signal tothe switches associated with each of the strings to be disconnected todisconnect the strings from the power converter. The means forconnecting the strings to adjust the photovoltaic voltage provided bythe photovoltaic generator comprises means for sending a control signalto the switches associated with each of the strings to be connected toconnect the strings to the power converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high level block diagram of a conventional photovoltaicsystem.

FIG. 2 is a more detailed block diagram of another the conventionalphotovoltaic system illustrated in FIG. 1.

FIG. 3 is a block diagram of another conventional photovoltaic system.

FIG. 4 is a block diagram of a photovoltaic system with a controllerconfigured to control strings of the photovoltaic generator.

FIG. 5 is a graph of grid voltage during a low voltage ride through(LVRT) event.

FIG. 6 is a graph of a number of connected strings resulting from afeed-forward response to the LVRT event.

FIG. 7 is a graph of PV voltage during disconnection of PV strings.

FIG. 8 is a graph of power sink current during disconnection of PVstrings.

FIG. 9 is a graph of power into a power sink of the system shown in FIG.4 during disconnection of PV strings.

FIG. 10 is a block diagram of a controller for the photovoltaic systemshown in FIG. 4.

FIG. 11 is a flow diagram of a method for regulating photovoltaicvoltage provided by a photovoltaic generator of the photovoltaic systemillustrated in FIGS. 4 and 5.

FIG. 12 is another block flow diagram of a method for regulating thephotovoltaic voltage provided by the photovoltaic generator of thephotovoltaic system illustrated in FIGS. 4 and 5.

FIG. 13 is a block flow diagram of a method for regulating the voltageprovided by the photovoltaic generator during a ramp up period of apower inverter of the photovoltaic system shown in FIG. 4.

FIG. 14 is a block diagram of a PV generator that can be used toimplement the PV generator illustrated in FIG. 4.

FIG. 15 is a block diagram of a PV generator that can be used toimplement the PV generator illustrated in FIG. 4.

DETAILED DESCRIPTION

Techniques are described for regulating the voltage provided by aphotovoltaic generator in a photovoltaic system. For example, aphotovoltaic system includes a photovoltaic generator (also referred toherein as a solar cell array) that includes photovoltaic cells arrangedin strings. A configurable string controller can detect events where thevoltage produced by the photovoltaic generator should be regulated andselectively connect or disconnect the strings to regulate the voltageprovided by the photovoltaic generator.

The controllable string combiner can monitor the power provided by thePV generator and determine whether the amount of power being provided bythe PV generator exceeds the capacity of a power sink or whether the PVvoltage exceeds a safety level of a power converter that converts the PVvoltage from DC to AC current. The power sink can be the electricalgrid, and the controllable string combiner can monitor the currentcapacity of the grid. The controllable string combiner can selectivelydisconnect the strings of PV cells to reduce the PV voltage if the PVvoltage exceeds the capacity of the power sink and/or exceeds a safetylevel associated with the power converter.

FIG. 4 is a block diagram of a photovoltaic system 400 that can be usedto implement the systems and methods described herein. A PV generator401 in the photovoltaic system 400 is divided into PV generator strings405 that can be selectively connected or disconnected from an powerconverter 402 (also referred to herein as an inverter) by a controller499 (also referred to herein as a controllable string combiner) in orderto control the PV voltage. Each string 405 comprises one or moreelectrically interconnected solar cells and, while each labeled thesame, may be different from each other.

FIG. 14 illustrates an example configuration of a PV generator that canbe used to implement PV generator 401 illustrated in FIG. 4. The PVgenerator comprises PV panels 1420 arranged into strings, with each PVpanel 1420 including one or more PV cells. Each string can include afuse 1405. The fuse 1405 can isolate a faulted string from the rest ofthe PV system 400 in the event that a fault occurs in the string.

A string comprises a series of electrically interconnected PV panels1420. A string can include a one-dimensional array 1455 of PV panels,such as the array 1455, and multiple one-dimensional arrays can becombined to form two-dimensional arrays, such as the array 1450. Fromthe perspective of the power converter 402, the set of interconnectedone and/or two dimensional arrays represents the PV generator 401.

FIG. 14 illustrates an embodiment where each switch 407 is associatedwith a two-dimensional array of PV panels and the controller 499 canoperate a switch 407 to connect or disconnect the two-dimensional arrayof PV panels associated with that switch 407. FIG. 15 illustratesanother embodiment of the PV generator 401 where each switch 407 isassociated with a one-dimensional array of PV panels, and the controller499 can operate a switch 407 to connect or disconnect theone-dimensional array of PV panels associated with that switch 407.

The amount of power that each of the strings can generate depends on theimplementation. For example, the number of PV panels 1205 included in astring can vary. For example, in some implementations, the amount ofpower generated by a string may range from 1 kilowatt (kW) to 3 kW. PVgenerators 401 can include many hundreds of individual strings.Additionally, a large 1 megawatt (MW) inverter could be used with a PVgenerator that includes hundreds of strings.

Returning now to FIG. 4, the power converter 402 converts direct current(DC) power provided by the PV generator 401 into alternating current(AC) power that can be provided to power sink 403. The power sink 403may comprise the electrical power grid and/or a microgrid that providesa localized grouping of electrical generation, storage, and loads. Forexample, the PV system 400 may be part of a microgrid that is designedto provide power for a university campus, an industrial complex, orother location where a localized generation of power is used to provideat least a portion of the electrical power.

The controller 499 is connected to switches 407. Each switch 407 isassociated with a string 405 of the PV generator and can be controlledby the controller 499 via a control connection 408. The controlconnections 408 can be either a wired or wireless connection that allowsthe controller 499 to send control signals to the switches 407 toconnect or disconnect the string associated with the switch 407. Each ofthe switches 407 can be a solid state relay that can respond to acontrol signal received via control connection 408 to connect the string405 associated with that switch 407 to the converter 402 or disconnectthe string 405 from the converter 402. For example, the switches 407 canbe implemented using solid state relays produced by Schneider Electric.Controller 499 can be configured to selectively connect or disconnectstrings until the capacity of the power sink or until the PV voltage islow enough and safe for the power converter.

While the system 400 includes three strings 405, different numbers ofstrings that make up the PV generator can be used. The greater thenumber of strings into which the PV generator is divided, the greaterthe level of granularity of control that the controllable stringcombiner 499 can have for adjusting the PV voltage. In finer-grainedsystems, where PV generator is divided into a larger number ofcontrollable strings 405, the controller 499 can make finer adjustmentsto the PV voltage by connecting or disconnecting the strings 405. Incontrast, in coarser-grained implementations, where the PV generator isdivided into a smaller number of controllable strings 405, thecontroller 499 can make more coarse adjustments to the PV voltage byconnecting or disconnecting the strings 405.

The controller 499 can be configured to monitor the capacity of thepower sink 403 and to react to changes in the capacity of the power sinkby temporarily disconnecting a number of strings of PV cells from the PVgenerator until the PV capacity increases. The controller 499 can alsobe configured to identify and respond to various types of events thatcan cause drops in the capacity of the power sink or increases in the PVvoltage output by the PV generator 401. Some examples include: (1) gridsupport operations during low voltage ride through (LVRT) events; (2)overproduction of PV power due to cloud edge effects; and (3) mandatorypower ramp up features during converter start-up period. These are justa few examples of the types of events to which the configurable stringcombiner can respond and are not intended to limit the use of theconfigurable string combiner to these specific events. The configurablestring combiner can be configured to respond to other types of eventsthat cause changes the in the PV voltage and/or the capacity of thepower sink.

As described above, the controller 499 can be configured to adjust thePV voltage during an LVRT event. An LVRT event occurs when the voltageof the grid is temporarily reduced. This reduction is typically due to afault or load change in the grid. During the LVRT event, the voltage maydecrease on one, two, or all three phases of the AC power grid. As thegrid voltage decreases, the controller 499 can selectively disconnectstrings from the PV generator to decrease the PV voltage. Once the LVRTevent passes, the controllable string combiner can reconnect thedisconnected strings as the grid capacity returns. The methodillustrated in FIG. 11 can be implemented by controller 499 and can beused for responding to an LVRT event.

The controller 499 can also be configured to adjust the PV voltage inresponse to cloud edge effects. Cloud edge effects can cause a suddenincrease in the PV voltage as the amount of sunlight reaching the PVgenerator increases as the cloud passes overhead. The controller 499 canselectively disconnect strings from the PV generator 401 to reduce thePV voltage to a level that is safe for the voltage converter and iswithin the capacity of the power sink. Cloud edge effects are typicallyshort-lived effect that produces spikes in PV voltage as an edge of thecloud passes over the PV generator 401. The controller 499 can beconfigured to monitor the PV voltage and to reconnect one or more of thedisconnected strings as the cloud edge effect passes. The methodillustrated in FIG. 12 can be implemented by controller 499 and can beused for responding to an increase in PV voltage resulting from cloudedge effects.

The controller 499 can also be configured to adjust the PV voltage byselectively disconnecting strings of PV cells from the PV generatorduring the startup period for the voltage converter. The voltageconverter may require an initial start-up period during which the PVvoltage must be ramped up gradually. The controller 499 can graduallyramp up the PV voltage by selectively connecting strings of PV cells ofthe PV generator to gradually increase the PV voltage during the startupperiod. The method illustrated in FIG. 13 can be implemented bycontroller 499 and can be used for gradually ramping up the PV voltageduring the startup period of the inverter.

FIG. 10 is a block diagram of a string combiner controller that can beused to implement controller 499 illustrated in FIG. 4. Controller 499includes a processor 1005, memory 1020, voltage inputs 1035, voltmeter1030, and control interface 1040. The memory 1020 includes a voltagecontrol module 1022, a string selection module 1024, and a controlsignal module 1026. The memory 1020 can comprise one or more types oftangible, non-transitory computer-readable memory, such as random-accessmemory (RAM), read-only memory (ROM), flash memory, or a combinationthereof. The modules can comprise processor-executable instructions thatcan be executed by processor 1005.

The processor 1005 can comprise one or more microprocessors configuredto access memory 1020. The processor 1005 can read data from and writedata to memory 1020. The processor 1005 can also read executable programcode from memory 1020 and execute the program code.

The voltage inputs 1035 provide an interface through which thecontroller 499 can monitor voltages throughout the photovoltaic system400. For example, the voltage inputs 1035 can be used to monitor thegrid voltage (V_(grid)) and/or the PV voltage (V_(PV)), the voltagegenerated by the PV generators. Voltmeter 1030 can be used to determinethe voltage of the various inputs being monitored using the voltageinputs 1035. The voltmeter 1030 may be an external voltmeter and thecontroller 499 can be configured to receive a signal from the externalvoltmeter that monitors the grid voltage and/or the PV voltage.

The processor 1005 can send control signals to one or more externaldevices via control interface 1040. For example, control interface 1040can be connected to control connections 408 that can be used to controlthe switches 407. The control interface 1040 can send a control signal1040 to a switch 407 associated with a particular string 405 of the PVgenerator 401 to connect or disconnect that string in order to controlthe overall PV voltage being provided by the PV generator 401. Controlinterface 1040 can be configured to provide wired connections, wirelessconnections, or a combination thereof for controlling the switches 407via the control connections 408.

The voltage control module 1022 can be configured to monitor the gridvoltage and/or the PV voltage to identify various events, such as LVRTevents, cloud edge effects, and/or other events and to respond to theseevents by selectively disconnecting or connecting strings of the PVgenerator to control the voltage provided by the PV generator. Thevoltage control module 1022 can be used to implement the methodsillustrated in FIGS. 11 and 12. The voltage control module 1022 can beconfigured to make a determination whether one or more strings of the PVgenerator 401 should be connected or disconnected. The voltage controlmodule 1022 can send a command to the string selection module 1024 todisconnect or connect one or more strings of the PV generator 401 inorder to regulate the PV voltage.

The control signal module 1026 can include executable code that cancause the processor 1005 to instruct the control interface 1040 to senda command to one or more external devices, such as the switches 407. Forexample, the control signal module can send a signal to a switch 407instructing the switch 407 to connect or disconnect the stringassociated with that switch. The control signal module 1026 can receivecommands from the string selection module 1024 to send commands to oneor more external devices, such as the switches 407.

The string selection module 1024 can be configured to select one or morestrings of the PV generator 401 to be connected or disconnected in orderto adjust the PV voltage provided by the PV generator 401. The stringselection module 1024 can also be configured to keep track of whichstrings are currently connected and which are currently disconnected.The string selection module 1024 can maintain a string map memory 1020that indicates whether each of the strings comprising the PV generator401 are connected or disconnected. The string selection module 1024 canbe configured to update the map as the strings are connected ordisconnected to adjust the voltage. The string selection module can alsobe configured to send a command to the control signal module 1026 toconnect or disconnect one or more strings of the PV generator 401 inorder to regulate the PV voltage.

FIG. 11 is a method for controlling the PV voltage output by the PVgenerator based on the grid voltage. The stages of the methodillustrated in FIG. 11 can be implemented by the voltage control module1022 of controller 499 unless specified otherwise. The methodillustrated in FIG. 11 can be performed by controller 499 whenresponding to an LVRT event. The method illustrated in FIG. 11 can beused by the controller 499 to respond to any changes in grid voltage andis not limited to just responding to LVRT events.

The grid voltage can be monitored to determine whether there are anychanges in the grid voltage that may require the PV voltage to beadjusted (stage 1105). The voltage control module 1022 of the controller499 can be configured to monitor the grid voltage.

The controller 499 can then make a determination whether the gridvoltage has decreased (stage 1110). If the grid voltage decreases, thephotovoltaic system can make a corresponding decrease in the PV voltageto reduce stress on the power converter 402.

If grid voltage decreased, the percentage of the grid voltage comparedto a nominal grid voltage can be calculated (stage 1115). The currentgrid voltage can then be divided by nominal grid voltage to determinewhat percentage of the nominal voltage the grid voltage is:

Percentage of nominal voltage=V _(GRID) /V _(NOM)

The nominal voltage represents a presumed voltage at which the grid isexpected to be operating. Certain events, such as LVRT events, can causethe grid voltage to decrease below the nominal voltage.

The percentage of the nominal voltage can then be used to determine anumber of strings to be disconnected (stage 1117). A percentage of thenumber of strings comprising the PV generator 401 to be disconnected canbe determined:

Percentage of strings to disconnect=100%−Percentage of nominal voltage

For example, if the grid voltage is 70% of the nominal voltage, then thegrid voltage is 30% less than the nominal voltage. Therefore, 30% of thestrings of the PV generator can be disconnected to reduce the PV voltageto 70% of the total PV voltage. The string selection module 1024 cankeep track of how many strings are currently connected or disconnectedand which strings are connected or disconnected. If the total number ofstrings that are currently disconnected (N_(disc)) is less than thetotal number of strings that should be disconnected (X_(disc)) based onthe decrease in grid voltage, the string selection module 1024 candetermine the number of strings to be disconnected (S_(disc)) bysubtracting the number of strings that are currently disconnected(N_(disc)) from the number of strings that should be disconnected(X_(calc)).

S _(disc) =X _(disc) −N _(disc)

If the number of strings to be disconnected is greater than zero, thecontroller 499 can send a control signal to the switches 407 associatedwith the strings 405 to be disconnected to cause the strings to bedisconnected (stage 1120). The string selection module 1024 can maintaina map of which strings are connected and which strings are disconnected,and can be configured to select connected strings to be disconnected,and can update the map of strings to reflect which strings have beendisconnected. The voltage generated by the disconnected strings is leftin the field with the solar array and is not introduced into theconverter. Once the strings have been disconnected, the controller 499can continue to monitor the grid voltage (stage 1105).

If grid voltage did not decrease, a determination can be made whetherthe grid voltage increased (stage 1130). If the grid voltage did notincrease, then the controller 499 can continue to monitor the gridvoltage (stage 1105).

Otherwise, if the grid voltage increased, the percentage of the gridvoltage compared to a nominal grid voltage can be calculated (stage1135). The current grid voltage can then be divided by nominal gridvoltage as described above to determine what percentage of the nominalvoltage the grid voltage is:

Percentage of nominal voltage=V _(GRID) /V _(NOM)

The grid voltage could increase after an LVRT event or other event thatcauses a drop in the grid voltage has passed. The controller 499 canreact to this change in the grid voltage by increasing the number ofstrings that are connected to allow the PV voltage to increase.

The percentage of the nominal voltage can then be used to determine anumber of strings to be connected (stage 1137). A percentage of thenumber of strings comprising the PV generator to be disconnected can bedetermined:

Percentage of strings to connect=100%−Percentage of nominal voltage

For example, returning to the previous example described above, if thegrid voltage changes from 70% of the nominal voltage to 90% of thenominal voltage, the number of strings of the PV generator that can beconnected can be increased from 70% to 90%.

The string selection module 1024 can keep track of how many strings arecurrently connected or disconnected and which strings are connected ordisconnected. If the total number of strings that are currentlyconnected (N_(conn)) is less than the total number of strings thatshould be connected (X_(conn)) based on the increase in grid voltage,the string selection module 1024 can determine the number of strings tobe disconnected (S_(conn)) by subtracting the number of strings that arecurrently connected (N_(conn)) from the number of strings that should beconnected (X_(conn)).

S _(conn) =X _(conn) −N _(conn)

If the number of strings to be connected is greater than zero, thecontroller 499 can send a control signal to the switches 407 associatedwith the strings 405 to cause the strings to be connected (stage 1140).The string selection module 1024 can instruct the control signal module1026 to send a control signal to the switches 407 associated with thestrings to be connected. The string selection module 1024 can maintain amap of which strings are connected and which strings 405 aredisconnected, and can be configured to select connected strings to bedisconnected and to update the map of strings to reflect which stringshave been disconnected. The voltage generated by the disconnectedstrings is left in the field with the solar array and is not introducedinto the converter. Once the strings have been disconnected, thecontroller 499 can continue to monitor the grid voltage (stage 1105).

FIG. 12 is another method for controlling the PV voltage output by thePV generator 401. The method illustrated in FIG. 12 can be implementedby the voltage control module 1022 of controller 499, unless otherwisespecified. The method illustrated in FIG. 12 can be implemented bycontroller 499 and can be used for responding to an increase in PVvoltage resulting from cloud edge effects. The method illustrated inFIG. 12 can be used by the controller 499 to respond to any changes inPV voltage and is not limited to responding to cloud edge effects.

The controller 499 can measure the grid voltage (stage 1205). Thevoltage control module 1022 of the controller 499 can be configured tomeasure the grid voltage. The grid voltage can be used to determine areference PV voltage.

The controller 499 can then determine reference PV voltage (PV_(ref))(stage 1210). The reference PV voltage represents a desired PV voltageto be provided by the PV generator 401 (the solar cell array). The PVvoltage can be determined by measuring the grid voltage and determininga PV voltage based on the grid voltage. The controller 499 can determinea desired PV voltage that would result in an output from the powerconverter 402 that has a voltage that matches or is close to the gridvoltage. The PV voltage can be predetermined and can be stored in memory1020 of the controller 499, and the voltage control module 1022 canaccess the reference PV voltage stored in memory 1020.

The controller 499 can then determine current PV voltage (PV_(curr))being produced by the PV generator 401 (stage 1215). As described above,the voltage control module 1022 of the controller 499 can be configuredmeasure the PV voltage.

The voltage control module 1022 can then be configured to determine thedifference between the reference PV voltage (PV_(ref)) and the currentPV voltage (PV_(curr)).

ΔPV=PV _(ref) −PV _(curr)

The voltage control module 1022 can then calculate the number of stringsto be connected or disconnected based on the difference between thePV_(ref) and the PV_(curr) (stage 1225). If the current PV voltage isgreater than the reference PV voltage, then the value of ΔPV will benegative, indicating that the controller 499 should disconnect one ormore strings to decrease the current PV voltage. If the current PVvoltage is less than the reference PV voltage, then the value of ΔPVwill be positive, indicating that the controller 499 should disconnectone or more strings to increase the current PV voltage. If the currentPV voltage is equal to the reference PV voltage, then the value of ΔPVwill be zero, indicating that no strings are to be connected ordisconnected.

The voltage control module 1022 can determine how many strings should beconnected or disconnected based on the value of ΔPV. For example, thevalue of ΔPV can be divided by a voltage production amount associatedwith each of the strings. Each of the strings may be assumed to producean equal estimated PV, and the ΔPV can then be divided by the estimatedPV to determine how many strings should be connected or disconnected.

Alternatively, the controller 499 can be configured to measure the PVgenerated by each of the strings and to select one or more strings thatcould provide approximately the ΔPV to be connected or disconnected.Where the generated PV voltage from each of the strings is measured, thestring selection module can be configured to periodically instruct thecontrol signal module 1026 to measure the PV generated by each of thestrings, and the string selection module 1022 can store this informationwith the map of connected and disconnected strings.

The voltage control module 1022 can then determine whether any stringsare to be disconnected based on the number of strings to be connected ordisconnected as determined in stage 1225 (stage 1230). For example, ifthe number of strings is negative, that number of strings should bedisconnected, and if the number of strings is positive, that number ofstrings should be connected.

If the number of strings to be disconnected is greater than zero, thecontroller 499 can send a control signal to the switches 407 associatedwith the strings 405 to cause the strings to be disconnected (stage1235). The string selection module 1024 can maintain a map of whichstrings are connected and which strings are disconnected, and can beconfigured to select connected strings to be disconnected, and to updatethe map of strings to reflect which strings have been disconnected. Thevoltage generated by the disconnected strings is left in the field withthe solar array and is not introduced into the converter. Once thestrings have been disconnected, the controller 499 can once againdetermine the current PV voltage (stage 1215).

If there were no strings to be disconnected, the voltage control module1022 can determine whether to disconnect any strings based on the numberof strings to be connected or disconnected as determined in stage 1225(stage 1225).

If no strings were to be disconnected, determine whether strings were tobe connected (stage 1240). If no strings were to be connected either,then the controller 499 can once again determine the current PV voltage(stage 1215).

Otherwise, if the number of strings to be connected is greater thanzero, the controller 499 can send a control signal to the switches 407associated with the strings 405 to cause the strings to be connected(stage 1245). The string selection module 1024 can maintain a map ofwhich strings are connected and which strings are disconnected, and thestring selection module 1024 can be configured to select disconnectedstrings to be connected and to update the map of strings to reflectwhich strings have been connected. Once the strings have beendisconnected, the controller 499 can once again determine the current PVvoltage (stage 1215).

FIGS. 5-9 are graphs of a simulated of an LVRT event and the response bythe string combiner controller 499. FIG. 5 is a graph illustrating adrop in grid voltage as a result of an LVRT event. During the event, thegrid voltage dropped to 30% of its initial value. FIG. 6 provides agraph that illustrates a feed-forward response (such as the methodillustrated in FIG. 11) to control the number of strings in response tothe LVRT event illustrated in FIG. 1. The number of connected strings isdecreased as the grid voltage decreases. For example, there wereinitially 16 strings connected before the LVRT event began. The numberof strings was stepped down to 6 strings as the grid voltage continuedto decrease. FIG. 7 illustrates that the PV voltage does not overshootas the load from the power sink (the grid) is removed. FIG. 8illustrates the current into the power sink (I_(grid)) the grid. As canbe seen in FIG. 8, the current into the power sink remains relativelyconstant as the LVRT event continues, because the controller hasselectively disconnected strings from the PV generator in response tothe decreased grid voltage. FIG. 9 illustrates the decrease in powerinto the power sink as the strings of the PV generator are selectivelydisconnected.

FIG. 13 is a method for regulating the voltage provided by thephotovoltaic generator during a ramp up period of the power inverter 402of the photovoltaic system 400. The power converter 402 may have amandatory power ramp up period while the power converter is beingstarted up. The controller 499 can implement a gradual ramp up of PVpower during this mandatory ramp up period. The voltage controllermodule 1022 can implement the stages of the method illustrated in FIG.13 unless otherwise indicated.

The controller 499 can be configured to receive an inverter startupsignal that causes the controller 499 to gradually ramp up the PV powerbeing provided to the power converter 402 (stage 1305). The inverter canbe configured to transmit a message to the controller 499 if theinverter is turned on or reset and requires a mandatory ramp up period.Alternatively, the inverter startup signal can be provided manually byan administrator or technician who has reset the power converter 402.The controller 499 may include a reset button or other interface thatallows the controller 499 to be manually reset into the inverter startupmode.

The controller 499 can then reset the PV output to a PV startup voltageby disconnecting all strings 405 of the PV generator in excess of the PVstartup voltage associated with the power converter 402 (stage 1310). Avalue representing the PV startup voltage can be stored in the memory1020 of the controller 499 and the voltage controller module 1022 canaccess that value to determine what the PV startup voltage should be forthat particular implementation. The voltage controller module 1022 cansend a command to the string selection module 1024 to connect therequired number of strings used to generate the PV startup voltage andto disconnect any strings in excess of the number required to generatethe PV startup voltage. The string selection module 1024 can select thestrings to be connected and/or disconnect and send commands to thecontrol signal module to connect or disconnect the strings as necessary.

The controller 499 can then iteratively connect strings to increase thePV voltage provided by the photovoltaic system 400 until a desired PVvoltage is reached (stage 1315). The voltage controller module 1022 canbe configured to add strings at a predetermined interval until thedesired PV voltage is reached. The voltage controller module maycalculate these increments based on a ramp up period duration definedfor the power converter 402, which can be stored in the memory 1020 ofthe controller 499 and access by the voltage control module 1022.Furthermore, the interval at which strings are added may be based atleast in part on the granularity of the system (the number of strings405 that comprise the PV generator 401 and the PV voltage generated byeach of these strings). The desired PV voltage may be determined usingthe methods illustrated in FIG. 11 or 12 or can be determined based onother factors, such as the current voltage of the power sink. Thisiterative process provides the power converter 402 time to ramp upwithout overloading and possibly damaging the power converter 402 duringthe startup period.

The various illustrative logical blocks, modules, and algorithm stagesdescribed may be implemented as electronic hardware, computer software,or combinations of both. To clearly illustrate this interchangeabilityof hardware and software, various illustrative components, blocks,modules, and stages have been described above generally in terms oftheir functionality. Whether such functionality is implemented ashardware or software depends upon the design constraints imposed on theoverall system. The described functionality can be implemented invarying ways. In addition, the grouping of functions within a module,block or stage is for ease of description. Specific functions can bemoved from one module or block without departing from the disclosure.

The various illustrative logical blocks and modules described can beimplemented or performed with a general purpose processor, a digitalsignal processor (DSP), application specific integrated circuit (ASIC),a field programmable gate array (FPGA) or other programmable logicdevice, discrete gate or transistor logic, discrete hardware components,or any combination thereof designed to perform the functions describedherein. A general-purpose processor can be a microprocessor, but in thealternative, the processor can be any processor, controller,microcontroller, or state machine. A processor can also be implementedas a combination of computing devices, for example, a combination of aDSP and a microprocessor, multiple microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The operations of a method or algorithm described may be embodieddirectly in hardware, in a software module executed by a processor, orin a combination of the two. A software module can reside in RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, or any other form of storage medium.An exemplary storage medium can be coupled to the processor such thatthe processor can read information from, and write information to, thestorage medium. In the alternative, the storage medium can be integralto the processor. The processor and the storage medium can reside in anASIC.

Various changes and modifications could be made to the descriptionsprovided above without departing from the scope of the disclosure or theappended claims. For example, although elements may be described orclaimed in the singular, the plural may be included. Additionally, allor portions of aspects and/or embodiments may be utilized with otheraspects and/or embodiments.

What is claimed is: 1.-9. (canceled)
 10. A method for controlling apower output of a photovoltaic system, the photovoltaic system includinga photovoltaic generator that comprises a plurality of strings, eachstring comprising one or more photovoltaic cells, the method comprising:monitoring a voltage of a power sink associated with the photovoltaicsystem; calculating a number of strings of the photovoltaic generator tobe connected or disconnected, based at least in part upon the voltage ofthe power sink, wherein disconnecting a string prevents power generatedby the string from reaching a power converter of the photovoltaic systemthat converts direct current (DC) power to alternating current (AC)power expected by the power sink and connecting the string allows powergenerated by the string to reach the power converter of the photovoltaicsystem; and connecting or disconnecting the number of strings to beconnected or disconnected to adjust the photovoltaic voltage provided bythe photovoltaic generator.
 11. The method of claim 10, furthercomprising: determining whether the voltage of the power sink hasincreased, and in response to the voltage of the power sink increasing:calculating the percentage of the voltage of the power sink relative toa nominal level associated with the power sink; calculating the numberof strings of the photovoltaic generator to be connected to increase thephotovoltaic voltage provided by the photovoltaic generator; andconnecting the number of strings to be connected to increase thephotovoltaic voltage provided by the photovoltaic generator.
 12. Themethod of claim 10 wherein the voltage of the power sink decreases as aresult of a low voltage ride through (LVRT) event, the method furthercomprising: reducing the voltage of the photovoltaic generator by anamount proportional to the decrease in the voltage of the power sink.13. The method of claim 10 wherein the voltage of the power sinkincreases after a low voltage ride through (LVRT) event, the methodfurther comprising: increasing the voltage provided by the photovoltaicgenerator by an amount proportional to the increase in the voltage ofthe power sink.
 14. The method of claim 10 wherein each of the pluralityof strings is associated with a switch, and wherein disconnecting thenumber of strings to be disconnected to adjust the photovoltaic voltageprovided by the photovoltaic generator further comprises sending a firstcontrol signal to the switch associated with each of the plurality ofstrings to be disconnected to disconnect the plurality of strings to bedisconnected from the power converter.
 15. The method of claim 14wherein connecting the number of strings to be connected to adjust thephotovoltaic voltage provided by the photovoltaic generator furthercomprises sending a second control signal to the switch associated witheach of the plurality of strings to be connected to connect theplurality of strings to be connected to the power converter.
 16. Aphotovoltaic system comprising: a photovoltaic generator comprising aplurality of strings, each string comprising one or more photovoltaiccells; a power converter configured to: convert direct current (DC)power provided by the photovoltaic generator into alternating current(AC) power; and output the AC power; a plurality of switches, eachswitch of the plurality of switches being associated with one of theplurality of strings and being configured to connect the stringassociated with the switch to the power converter when set to a firstsetting such that power generated by the string can flow to the powerconverter and to disconnect the string from the power converter when theswitch is set to a second setting; and a controller configured tocontrol a power output provided by the photovoltaic generator byselectively connecting or disconnecting one or more of the plurality ofstrings of the photovoltaic generator to the power converter, thecontroller being configured to monitor a voltage of a power sinkassociated with the photovoltaic system, calculate a number of stringsof the photovoltaic generator to be connected or disconnected, based atleast in part upon the voltage of the power sink, wherein disconnectingthe string prevents power generated by the string from reaching thepower converter of the photovoltaic system that converts direct current(DC) power to alternating current (AC) power expected by the power sinkand connecting the string allows power generated by the string to reachthe power converter of the photovoltaic system, and connect ordisconnect the number of strings to be connected or disconnected toadjust the photovoltaic voltage provided by the photovoltaic generator.17. (canceled)
 18. The method of claim 10 wherein calculating a numberof strings of the photovoltaic generator to be connected or disconnectedcomprises: determining a reference voltage for the photovoltaicgenerator based at least in part upon the voltage of the power sink, thereference voltage representing a desired voltage for the photovoltaicgenerator; determining a current voltage of the photovoltaic generator;determining a difference between the reference voltage and the currentvoltage; and calculating the number of strings of the photovoltaicgenerator to connect or disconnect based on the difference. 19-20.(canceled)
 21. The photovoltaic system of claim 16 wherein thecontroller is configured to calculate the number of strings to beconnected or disconnected by being further configured to: determine areference voltage for the photovoltaic generator based at least in partupon the voltage of the power sink, the reference voltage representing adesired voltage for the photovoltaic generator; determine a currentvoltage of the photovoltaic generator; determine a difference betweenthe reference voltage and the current voltage; and calculate the numberof strings of the photovoltaic generator to connect or disconnect basedon the difference.
 22. (canceled)
 23. The method of claim 10, furthercomprising: determining whether the voltage of the power sink hasdecreased, and in response to the voltage of the power sink decreasing:calculating the percentage of the voltage of the power sink relative toa nominal level associated with the power sink; calculating the numberof strings of the photovoltaic generator to be connected to decrease thephotovoltaic voltage provided by the photovoltaic generator; anddisconnecting the number of strings to be connected to increase thephotovoltaic voltage provided by the photovoltaic generator.
 24. Themethod of claim 18 wherein determining a reference voltage for thephotovoltaic generator based at least in part upon the voltage of thepower sink includes accessing a predetermined photovoltaic voltagestored in memory.
 25. The photovoltaic system of claim 16 wherein thecontroller is further configured to: determine whether the voltage ofthe power sink has decreased, and in response to the voltage of thepower sink decreasing: calculate the percentage of the voltage of thepower sink relative to the nominal level associated with the power sink;and calculate the number of strings of the photovoltaic generator to bedisconnected to decrease the photovoltaic voltage provided by thephotovoltaic generator, based upon the percentage.
 26. The photovoltaicsystem of claim 16 wherein the controller is further configured to:determine whether the voltage of the power sink has increased, and inresponse to the voltage of the power sink increasing: calculate thepercentage of the voltage of the power sink relative to the nominallevel associated with the power sink; and calculate the number ofstrings of the photovoltaic generator to be connected to increase thephotovoltaic voltage provided by the photovoltaic generator, based uponthe percentage.
 27. The photovoltaic system of claim 16 wherein inresponse to a decrease in voltage of the power sink as a result of a lowvoltage ride through (LVRT) event, the controller is further configuredto reduce the voltage provided by the photovoltaic generator by anamount proportional to the decrease in the voltage of the power sink.28. The photovoltaic system of claim 16 wherein in response to anincrease the voltage of the power sink after a low voltage ride through(LVRT) event, the controller is further configured to increase thevoltage provided by the photovoltaic generator by an amount proportionalto the increase in the voltage of the power sink.
 29. The photovoltaicsystem of claim 16 wherein each of the plurality of strings isassociated with a switch, and wherein disconnecting the number ofstrings to be disconnected to adjust the photovoltaic voltage providedby the photovoltaic generator further comprises sending a first controlsignal to the switch associated with each of the plurality of strings tobe disconnected to disconnect the plurality of strings to bedisconnected from the power converter.
 30. The photovoltaic system ofclaim 29 wherein connecting the number of strings to be connected toadjust the photovoltaic voltage provided by the photovoltaic generatorfurther comprises sending a second control signal to the switchassociated with each of the plurality of strings to be connected toconnect the plurality of strings to be connected to the power converter.31. The photovoltaic system of claim 21 wherein determining a referencevoltage for the photovoltaic generator based at least in part upon thevoltage of the power sink includes accessing a predeterminedphotovoltaic voltage stored in memory.